Innovations and Fundamental Insights of Advanced Field Emission Cathodes for High Power Microwave (HPM) Sources

One of the key remaining challenges for HPM (> 100 MW, <1 mus) sources is the development of robust high current density field emission cathodes. Funded by the USDOD/AFOSR under the MURI04 program, a university consortium, in collaboration with other institutions, has achieved significant progress on identifying promising cold cathode candidates and understanding the basic physics that give them their unique emission characteristics. Cathodes being investigated include micro-ridged monolithic-metallic cathodes, Csl-coated graphite fiber cathodes, and, a recent innovation: micro-patterned, metal-oxide-junction triple-point cathodes. Cs-I-coated C cathodes have been observed to have low turn-on electric fields and negligible evidence of cathode plasma. At high voltage, monolithic-metal cathodes have demonstrated currents up to 80 kA, current densities > 100 A/cm² and negligible evidence of cathode plasma. Experimental I-V characteristics of monolithic-metal cathodes suggest anomalously high local surface electric field enhancements (> 100) combined with small fractions ( 0.01) of the cathode surface responsible for the emission. These collective observations raise fundamental questions about the mechanisms and limitations of the electron emission. This talk summarizes the findings on novel cathode performance, emission mechanisms, the role of surface nano-and micro-scale properties, and emission constraints posed bv the space-charge-limited Fowler Nordheim condition.